The administration of CPAP is common in the treatment of Obstructive Sleep Apnea (OSA) syndrome and Upper Airway Resistance syndrome. CPAP treatment effectively acts as a pneumatic splint of a patient's upper airway by providing air or breathable gas at a pressure elevated above atmospheric pressure to the entrance of the patient's airway. Treatment pressures in the range 4-20 cm H.sub.2 O are commonly encountered. More sophisticated forms of CPAP include bi-level CPAP in which different treatment pressures are applied in synchronism with the inspiratory and expiratory phases of respiration, and autosetting (controlled variable treatment pressure) CPAP, as described in U.S. Pat. No. 5,245,995. In all forms of CPAP treatment it is desired to maintain the treatment pressure, usually clinically determined by a physician, to be as constant as possible to maintain treatment efficacy without causing the patient undue discomfort by having to work against an unnecessarily high positive airway pressure.
Common to all forms of CPAP apparatus is a mask worn b a patient having connection via a flexible air delivery tube to a flow generator. The flow generator has a turbine driven by an electric motor that is under the control of a microprocessor-based controller.
In this specification a reference to a "mask" is to be understood as including a nose mask, a mouth mask, a nose and mouth mask in combination or a full face mask.
Simple CPAP machines estimate the mask pressure from the motor speed and operate under speed regulation. More sophisticated machines incorporate a pneumatic pressure transducer that provides a feedback signal representative of pressure at either the mask or a point within the flow generator itself. If the pressure feedback signal is from the mask, additional tubing or wires must extend between the mask and the flow generator which can give rise to sterilisation and/or safety problems. If pressure feedback is from a point within the flow generator, or at some other point removed from the mask, the impedance of the air delivery tube can result in flow-induced pressure swings at the mask. The pressure swings can be up to +5% of treatment pressure, and since it is desired to provide the patient with the minimum treatment pressure to provide treatment efficacy and yet avoid the patient doing unnecessary work during exhalation, these pressure swings are undesirable and should be eliminated as far as possible.
It is also desirable to be able to accurately determine dynamic air flow in the delivery tube and mask. "Flow" is to be understood as including both ventilation volume and a volumetric flow rate. The flow will have a component due to the flow generator that is modulated by patient respiration. The measurement of air flow in the air delivery tube can beneficially be used to measure the average volume breathed by the patient and to determine whether the patient is inhaling (inspiring) or exhaling (expiring), the latter of which is crucial in the implementation of bi-level CPAP. Currently this is done using an in-line sensor to directly measure flow, or by measuring the pressure drop across a restriction in the air delivery tube (or alternatively, the pressure drop alone the air delivery tube). These methods require the use of additional transducers, and in some cases additional wiring or tubing to connect the transducer to an appropriate point in the control circuitry of the CPAP apparatus.
It thus also is desirable to accurately measure or estimate air flow from the stand-point of enabling advanced control over the administration of CPAP treatment and ensuring efficacy of treatment and patient compliance without reducing patient comfort.
The present invention is directed to overcoming or at least ameliorating one or more of the foregoing problems.